text:
Why are airplanes described as the safest method of transportation?
I will have my first flight tomorrow. It is my first time that I get into an airplane, and by the time am  writing this post, I am feeling anxious and stressed a lot. I even don't talk to anyone as it is the last day of my life and need to live it alone. I read that this is called "fear of flight" or "phobia of airplanes".
I read online on all related sites, that the airplanes are the safest transportation mode. I could die with a car accident while I am getting to airport 20 times more than be killed in an airplane crash.
All sites said that I should understand more about airplanes, and that would release the tension and make me better.
I am thinking to cancel the flight and the 330$ will go through the wind as we say in my country, and not be refunded. I read a lot of things but all are technical, and I didn't understand any single word, especially that I am a person that don't have any mechanical or technical background to understand it.
My doctor told me to start taking "Xanax" and that would help me be fine on my less than 2 hours flight (most of it is over the sea that's why I am more anxious).
Why are airplanes the safest method of transportation? Why aren't ships the safest? Because in 3 years I didn't hear of an airplane that sank.

Welcome to the site! It is really difficult to answer your question in the format intended for this site, because there is not a single simple answer to why flying is so safe. Safety is the number one concern within all sectors of aviation, and the statistics speak for themselves. But to explain why flying is so safe in just a couple of paragraphs, without getting technical, would be very difficult.

There is no need to understand the reasons that make air travel safe, statistics are speaking. Regarding ships, though you may find a counterexample, it's possible to continue breathing and take your time to land after a plane is depressurized due to a leak, but you can sink when you are at 500 NM from the coast and there is a hole in the hull (just one example). If you accept the idea that superstition has no valid grounds, then it's the same for fear of flying.

You may also find the comments on this closed question useful: https://aviation.stackexchange.com/questions/22116/how-to-control-fear-of-flying

This looks like a duplicate of the question shared by Cody, and I think it is off-topic also this one

You can never switch of fear or phobia, but I hope you overcome it enough to take your flight. Flying is an awesome experience, and a privilege which we as humans have only been able to achieve in the last few seconds of our evolutionary life. If you do overcome it, you will be able to visit places which you can only dream about.

@Jamiec: " you can't switch of[f] a phobia" ? Fear of flight happens to be eminently treatable, many people are cured within one day. And that is for a durable treatment.

@Jamiec Yeah. This is the key. Overcoming the phobia. It need more power and self confidence to do it. I am not traveling alone. I am going with friends too and they are used to it and very excited. I think this is what keeps me thinking that everything going to be fine lets hope. Thanks all.

What intimidate me is that the percentage of accident is low like 0.001%, but when it happens, the percentage of losing all passengers their lives is 100%. No accidents had survivors.

@MSalters what is this treatment ?

@droidnation: It's a form of Cognitive behavioral therapy (CBT). This is a therapy used for many phobias. But since fear of flying is fairly common, there are a number of practitioners who have specialized.

@droidnation: The only major accidents with a major carrier in 2016 was Emirates Flight 521; all 282 passengers and 18 crew members survived, and Singapore Airlines Flight 368, 222 passengers, 19 crew, also no injuries. (I assume you're not flying in Africa.) In 2017, the biggest accident with passengers planes was Peruvian Flight 112, 39 injuries but none fatal. BTW, that 0.001% is a gross overestimate - that's close to 10 per one million flights. Major accidents are about 100 times less frequent, 0.00001% or one in ten million flights.

Re: CBT, be careful to pick a licensed psychotherapist. As administered by a professional dominatrix, CBT could leave you with two phobias.

@droidnation you're really wrong. The survival rate from all classes of incident on board an airline is crazily high like 99.9% or something. Obviously nobody survives a mid air explosion sort of incident but those are vanishingly rare.

The difficulty is mixing up the chance of having an accident while using a plane with the effect that accident will have should it occur. A car crashing into something is rarely fatal for the passengers because cars are mostly slow when this sort of thing happens. When they are not, well then a fatal outcome is more likely. Planes are very very fast compared to cars, so any crash a plane is involved in is likely to be very much fatal. However, planes collide so rarely with anything compared to the number of passengers carried, that your chance to die in a plane accident is lower than in a car.

@Adwaenyth Even that isn't really true. Planes hit birds fairly frequently, but it only very rarely causes a problem for the plane--and even then the plane usually lands with all passengers safe (the birds unfortunately aren't so lucky). Cars vs deer, on the other hand, cause a lot of human fatalities.

This question is not off-topic, or a duplicate. It's asking a question about aviation. It's true that the question it asks isn't really the question that the author needs answering, but that doesn't make it off-topic. It just requires that the question be understood with a certain amount of human empathy to grasp what it's really about. It's a question that in different forms gets asked here occasionally and deserves to be answered. If this forum is unable to address questions about fear of flying adequately, then it really is in danger of falling victim to its own pedantry.

So, @droidnation, how was your first flight in the end?

@Jamiec "Obviously nobody survives a mid air explosion sort of incident" https://aviation.stackexchange.com/questions/44129/why-are-airplanes-described-as-the-safest-method-of-transportation/44140#comment113771_44135

@FabioTurati it was awesome. And Istanbul was awesome too despite the bad weather. We had some turbulence but it was really good.

Great! The next step is to start collecting miles until you are a frequent traveller! :-)))

@FabioTurati I collect miles from the company that I traveled with right ?

Yes, but consider that in general you need to fly quite often if you want to achieve the status of frequent traveller. A return flight every month is probably not enough. Signing up, in any case, is free, so you have nothing to lose.

@FabioTurati thanks Fabio. I can send you some pictures about my trip by email. Your words encouraged me to take the flight. You should work on people with airplane fears and help them. Thanks again.

Good news
The good news is: you really are safe in an airliner, so safe that you could spend every minute of every day of the the rest of your life flying in one, and still not face any greater meaningful risk of a disaster.
It's difficult to answer your question why it's so safe (i.e. what makes it safe), but the record is there, plainly, that it is so safe.
Bad news
The bad news is: if one has an irrational fear of flying, then no objective facts or understanding will help lessen it.
If the fear is based on misunderstanding or lack of knowledge, then of course learning more will help overcome it. However, that's not the basis of a phobia, or the kind of fear you seem to be describing.
Advice to understand flight and aircraft is well-meaning, but not actually that useful. Someone can have an excellent understanding of aviation
engineering and aerodynamics and all manner of subjects, and know that it is exceptionally safe, and still suffer from crippling fear of actually flying.
Everyone is different, but such fear is often brought on by an event or change in life that has nothing whatsoever to do with flying, but somehow gets expressed in a fear of flying.
The comfort and support of another person on board could be the most valuable thing you seek before your flight. I'd suggest expressing your fear, without embarrassment or shame, to the staff at the gate. They and the crew on board will have helped people in such cases many times before, and they are good at it.
Simply getting used to an object of fear is one of the best ways of dealing with it.
I hope you make your flight, and I hope to hear that having done it once, the fear diminished by being stared down, and that subsequent flights render it lesser still. Please tell us about it here.

Sure I will. Thanks for your answer.

And a reasonable dose of Xanax should help to relax as well in many cases.. (though I cannot recommend to do it regularily)

What intimidate me is that the percentage of accident is low like 0.001%, but when it happens, the percentage of losing all passengers their lives is 100%. No accidents had survivors.

@droidnation Not true. Remember that time a few years ago when things went wrong on a flight but the pilot managed to safely land the plane along the hudson river? Everyone survived that.

@Pyritie But not every plane is designed to land on the sea or river right ?

@droidnation: In fact every commercial airplane that flies over the sea is designed to be able to safely land on the sea. Each passenger even has his own life vest stored under his seat, and the airplane is equipped with rafts.

Good answer. Could expand on the statistical side in the good news part. For example (from here http://theweek.com/articles/462449/odds-are-11-million-1-that-youll-die-plane-crash) the chances of an incident in a flight are 1 in 1.2 million, and 95% of people survive those incidents. You're probably more likely to randomly drop dead than be killed by the plane.

Appreciate that guys. Thanks.

"when it happens, the percentage of losing all passengers their lives is 100%." - absolutely not. On the contrary, the majority of accidents have no casualties whatsoever! Planes have landed safely with no fuel, engines on fire, broken chassis, etc.

"when it happens, the percentage of losing all passengers their lives is 100%."  This is completely untrue.  Consider the flight Qantas 32: They had an engine literally explode, over the ocean, with a full load of fuel on board.  It was truly a worst-possible-case scenario.  Yet they still landed the plane, and not a single person was even injured, much less killed!

Heck, even midair explosions (now that's a worst case scenario!) have had multiple survivors.

@droidnation: Modern airliners are built to withstand multiple kinds of failures and still land safely, and crews are trained to keep you safe and alive in the event of an emergency.  There really are very few ways for things to go so wrong such that any passengers are lost.  It's okay to be a little nervous on your first flight, but take it from someone who's clocked their fair share of hours in the sky - flying is boring.

Guys...I did it. It was awesome. We had some turbulence but it was really good.

I did it. FINALLY. Istanbul was great. All of your answers and comments really helped me. Thanks all.

I have the exact same problem as you described. Congratulations that you've overcome your fear of flying! Since then how many flights have you done?

Ships are subject to storms, aircraft fly above them. Cars are confined to a very crowded space called a road, and can collide with all the other cars sharing the road - aircraft have the infinite skies.
Understand an aircraft - they are very competently engineered constructions and are indeed the safest form of transport. Accidents are mainly caused by the unknown, and for over half a century aeronautics has mapped previously unknown factors into the world of understanding. There is very little unknown randomness in aviation.
On a 2-lane road, you see car after car coming toward you, driven by a person who may be distracted or sleepy, only 50 cm away from a collision. Yet you never think about it because you are used to it. Familiarity generates acceptance: things become unexciting.
You're taking your first flight and are not familiar with flying and now must trust us, the engineers who made the plane and the pilots who drive it. We're rational people. It is fear of the unknown that you are experiencing, however it is not unknown to us. Don't concentrate on the distraction (it is unknown) but concentrate on the boring safety record of aviation. Watch birds fly, they don't fear it. And ultimately it is like Franklin D. Roosevelt said: you have nothing to fear but fear itself.

Pedant alert. The sky is by no mean infinite.

To add on Jamiec, you might want to call them unlimited, and East-West & North-South it is true, but not so much in the Up-Down direction, especially down.

Second paragraph doesn't exactly work better for me. Calling devices "incredibly clever" is a sure fire way to kill my faith in its reliability. But luckily, accidents are mainly caused by the unknown :) The last paragraph salvages the whole post.

Good to know. I find the amount of risk mitigation and catering for the unknown applied to aircraft very comforting. Risk reduction is a science. I did not realise that the word "clever" is associated with increased riskiness.

@Jamiec Travel ever westwards and thou won't find finity.

You nailed the point that makes flying uncomfortable for me. A car can crash, a ship can be caught by a storm, but if the airplane fails (engine outage, collision with other aircraft, whatever) during the flight there is this thing called gravity. And if gravity takes major control of the flight path there is a chance close to a guarantee you won't make it out alive.

@Koyovis you won't find a limit, but it is finite (you can measure the area/volume -> finite, but you can't say where it starts or ends -> unlimited)

What intimidate me is that the percentage of accident is low like 0.001%, but when it happens, the percentage of losing all passengers their lives is 100%. No accidents had survivors.

@droidnation Yes the consequences of an accident are very visible. The great news is that you have a chance of zero to be in one. Really, a lot less than 0.001%. More like 0.000001% per flying hour. Spend 114 years in an aeroplane seat and have a chance of 1 in a thousand of dying in a plane crash. Spend more than a thousand lifetimes continuously in a seat to have a serious chance of crashing.

@Pat Airplanes are able to fly with one engine out of operation. (Even 1-engine aircraft can still glide to land, but that's very likely not what the OP is getting into.) In fact, IIRC having full (albeit reduced performance) flight ability with one engine out is a regulatory requirement for multi-engine airplanes. The reason, of course, being that even though in-flight failures of critical subsystems (such as one of the engines) is rare, it can still happen. Hence regulations bodies (such as FAA or EASA) enforce requirements to ensure the ability to safely land even when they do happen.

I even don't talk to anyone as it is the last day of my life and need to live it alone. I read that this is called "fear of flight" or "phobia of airplanes".

No it is not, in fact its going to be the first day when you learn new heights you can go to, you will make new friends if you're good at it. You will see new views from the Sky. You will see a new perspective to life.
And you will be able to enjoy 800+ kph ground speed while still being able to walk around in an aisle as if it was nothing.
Im a frequent flier but every time I fly airport operations and the sophistication of the aeroplanes never fail to amaze me. Enjoy your tour to one of the best machines man has ever produced!
Its ok to be afraid, it is also ok to admit it but the real key with dealing with any fear is to try to control it for a few moments of decision time, after that momentum takes over and by the time you realize your fear is long gone.
You only have to control your fear till the time your mind is telling you to cancel the ticket, if you keep that in control and reach the airport then the series of steps leading to your journey and arrival at the destination will keep you so immersed in the process that you won't even remember you feared this. Its such a nice process.
Relax, millions of people enjoy that journey everyday. After your flight most probably your only complain will be the quality of in-flight meal :P
Look forward to it!

What intimidate me is that the percentage of accident is low like 0.001%, but when it happens, the percentage of losing all passengers their lives is 100%. No accidents had survivors.

Don’t bother. They are nothing more than stats. If something has to happen it can happen in the bedroom as well.  Who likes to be part of an accident with even 99 percent survival chances? Nobody wants to be part of any accident good or bad

Ironically air travel is safe BECAUSE you are so afraid of it ;-).
A big contribution to the safety of air travel is that people are so fearsome about it. You are not the only one - being confined to a metal tube in an environment where you could not breath and would freeze to death at the same time in the matter of seconds without the continuous operation of life supporting technology, while at the same time sitting on several tons of highly flammable liquid is something that will make only the most unimaginative people comfortable, even though the technology that deals with those risks has become very reliable.
Still, have a look at news coverage of any aviation disaster. Coverage is usually extremely extensive. It damages the reputation of the airline, the manufacturer and possibly the whole aviation industry, because people are so afraid. The response of the modern (western) aviation industry is a commitment to safety that goes far beyond the standards of most other industries. If something goes wrong, then there is (seemingly) no shortage of money to find out what exactly went wrong and what can be done so that this never happens again. Think about the Air France Flight 447 which crashed into the sea in a very remote location. I don't have exact figures, but just by reading through the reports of the search&rescue operation with all the highly specialized equipment you can already guess that the industry spent many, many millions of dollars just to find out what was wrong. The reasons for the accident were discovered, lessons were learned, technology was improved and pilot training was amended, just so that this won't happen again.
Air travel has not always been as safe as it is today. But there were many lessons learned. There is never a "single point of failure". E.g. if a twin engined airplane looses one engine during take-off, it will either be able to stop before the end of the runway or it will be able to take-off on the remaining engine and return for landing safely. This is calculated before each and every take-off and the airplane won't leave the gate if calculations show that this is not possible. Modern jets have at least two, most three, some even four fully independent steering systems. On 22 November 2003 a DHL frighter plane was hit by an ant-aircraft missile in Bagdad. Think about it: This was a military attempt to bring down an airplane. This missle hit it's target, the planes wing was on fire and the plane lost ALL it's redundant steering systems. Nevertheless the pilots managed to land the crippled plane in Bagdad. This shows that even with the complete loss of multiple systems planes are nowadays so well engineered, that it is even hard to crash them on purpose. This is something that terrorists also learned on 11 December 1994 when they planted a bomb on Philippine Airlines Flight 434 which blew up and nevertheless that 747 did not fall apart but made a successful emergency landing.
Airplanes are built to withstand things you never want to live through. They are built to land on water, to land without the landing gear, to continue to fly without fuel and engines. They will continue to operate if the cabin door falls out in mid-flight (you'll get cold and possibly suffer from hypoxia and need to get some treatment afterwards, but you'll survive without any permanent damage).
Nevertheless airplanes are not invincible as it has been demonstrated in history. We know that fire on board is a problem so during certification it must be shown that an aircraft can be completely evacuated in 90 seconds through only half of the doors and materials used in construction must not be easily inflammable. Also pilot training plays a critical role and pilots are trained to avoid risks and always put safety concerns first. Nowadays pilots are trained for even the remotest scenarios: Computers which finally allow the operation of highly realistic flight simulators allow to do training that has simply not been possible a couple of decades ago. On the dark side pilot training is expensive as hell. Actually just operating a modern training simulator is expensive as hell, but it is all "safety first" and this is also because the whole aviation industry knows that a single accident might end in bankruptcy.
Still nothing is 100% safe. Not even lying in bed is safe - the building could collapse on you. And it is not equally safe to fly everywhere in the world. Flying up far north or far south is "easier" than flying in the tropics where weather is much more severe, but most importantly not all countries and airlines share the "safety culture". I'd not feel comfortable with some airlines in certain developing countries either, but you can feel very safe with western airlines (also when they operate in the tropics).
Edit (concerning Europe)
I see that you are from Germany and you wrote your flight is about 2 hours. So you will basically fly over Europe the entire time. Specifically for that area flying is very safe. Computer assisted weather forecasts and advanced weather radars on board airplanes have made weather much less dangerous than it used to be, but it is still of concern. However the dangers over Europe are very very low. Weather is not nearly as severe as in the tropics and things like thunderstorms are usually local and can be flown around. Even if you cannot reach your destination due to weather there are probably 10-20 possible alternate airports on your route, so pilots will divert if weather is a problem. Also in case of any emergency there are always suitable places to land (I have been on a plane where a passenger had a sudden life threating health condition and 15 minutes later we were from cruising altitude on the ground). Also Europe has generally a good safety culture and airlines that are considered unsafe are blacklisted and won't fly in Europe. Europe has a very dense airspace with approximately 1 billion (!) passengers reaching their destinations safely each year. Nevertheless accidents are seldom.
Edit 2 (sharing of learning)

If an accident happened with Air France, does the results are shared with other airline companies like Qantas, and even the small ones like Atlas Global per example ?

Yes these results are shared in details. Not only with other airlines, but even with the public. E.g. the report concerning the Air France crash can be     downloaded here. It has 223 pages and goes into a lot of details about exactly what happens and includes recommendations for all airlines world wide to prevent these things from happening again. The recommendations are often picked up by local authorities like the EASA (European Aviation Safety Agency) or the FAA (Federal Aviation Agency (USA)) who then withdraw the airworthiness of planes that have not upgraded the airplanes. E.g. as part of the aftermath from the Air France accident:

On 12 August 2009, Airbus issued three Mandatory Service Bulletins, requiring that all A330 and A340 aircraft be fitted with two Goodrich 0851HL pitot tubes and one Thales model C16195BA pitot (or alternatively three of the Goodrich pitot tubes); Thales model C16195AA pitot tubes were no longer to be used. This requirement was incorporated into Airworthiness Directives issued by the European Aviation Safety Agency (EASA) on 31 August and by the Federal Aviation Administration (FAA) on 3 September. The replacement was to be completed by 7 January 2010. (Source: Wikipedia)

In Germany the BFU (Bundesstelle für Flugunfalluntersuchung) is dedicated to investigating accidents and incidents. They publish their findings here. You'll find that nearly all incidents and accidents are related to private pilots and indeed it is much safer to fly with a commercial airline which have better training, better maintenance and more experience than most private pilots. Nevertheless even those accidents are investigated in detail for the aviation world to learn from.
Note that there even things you'd never noticed as a passenger are already regarded as an incident. E.g. by law each flight must be planned so that the plane as enough fuel to fly to its destination airport, then continue to fly to another airport and land there with 30 minutes of fuel to spare. If a plane lands anywhere with less than 30 minutes worth of fuel in the tank, than this is already an incident which will be officially investigated to find the causes and help the aviation world to learn and avoid these things in the future.

Thanks Yankee. Just a question. If an accident happened with Air France, does the results are shared with other airline companies like Qantas, and even the small ones like Atlas Global per example ?

@droidnation Not only are accident reports completely public; if it is found that the cause could affect other airplanes, they are no longer allowed to fly until the problem is fixed (airworthiness directives).

@droidnation: Yes. There is a very comprehensive sharing of safety information. In particular, if there is an issue with a plane (which is generally just an annoyance as pretty much everything is redundant), Air France will report it to the manufacturer, and the manufacturer will look up every other owner of that model and share the information. And the same applies to Atlas Global. In fact, the manufacturer doesn't just keep track of the planes they sold, but of each and every individual part on that plane. 6th tire on the left? The manufacturer knows the exact time and date it was made.

Adding to what @MSalters wrote, if there's some reason to believe that, say, all tires from a specific manufacturing batch has the same fault, the manufacturer can use that same information to track down which other operators and aircraft might be affected.

"They will continue to operate if the cabin door falls out in mid-flight" And the door is typically attached in such a way that lots of things have to go very, very wrong before it "falls out in mid-flight". The lower air pressure on the outside of the aircraft at altitude means that there is a huge force acting on the door from the inside; the door opens to the inside, which means that all of that air pressure is keeping the door shut rather than trying to force it open in the normal direction of movement. (We do have a question or two about deliberately opening jetliner doors in flight.)

@droidnation further to what others said, not only can they tell you the serial number of every part on the aircraft they can track everything that has ever happened to every part from raw materials. Take a blade for example, they can trace exactly what batch of material it came from, the machine it was made on, who operated the machine, who fitted it, who tested it to make sure it was safe, how many hours it's been in use. And all that data is kept a minimum of 40 years. That volume of data is just one of the many things in Aviation that helps keep you safe. It's an industry like no other.

@droidnation: I added another edit to my answer to answer your comment.

In my view planes are saver transportation then all others mainly because of this reasons:
Physical force range
Air produces limited force attacking the fuselage of a plane. There is no such phenomenon like freak waves which can damage shippes, or rocks laying on the road after a bend destroungin a car hitting it.
The only phenomenon able to destroy a plane in flight is another plane or an extraordinary strong thunderstorm. Both can be easily detected and avoided.
In flight communication.
Radio
Pilots have (almost) constant possibility to talk to a controller on the ground via radio. Since this is a shared medium all other pilots in the same area (communication with the same controller) get to know what the other plains are doing or have been told to do. This way each pilot knows about the intentions of the other pilots around him.
Anti Collision System
Commercial aircrafts have an anti collision system which also communiates via radio and informs the pilot about other aircrafts which are close by and potentionally on a collision curse.
Maintanence
Check Schedule
For each aircraft type there is a detailed check schedule that defines for each part or the plane after how many working hours it needs to be inspected. This schedule contails simple visual checks (in shorter intervalls) and more elaborate checks where the part needs to be disassembled and/or inspected using tools like ultra sonic devices and similar (in longer intervall).
Pre Flight check
As a part of the check schedule the crew must perform a preflight check which involves visual inspection of the plane from outside and functional checks of the most important systems.
(I usually aks my passengers when they last checked the oil or the break fluid in the car they came with...)
Fail Safety
The most important systems of a comercial airplane are backed up. That means you have independend system doing the same.
And most importand: any plane can be landed safely as long as you can control speed and direction, even if all engines fail.
Crew Training
The crew regularily trains the emergency procedures and handling of the plane when certain technical problem occure. This ensures almost ever (at least) a safe landing if a system of the plane fails during flight.
Accident Investigation
Although this is my last point this is the most important one!
After an accident its root cause is investigated.
The results of this investigation is transfered into either

regulations on how to build aircrafts,
orders to the manufacturer to update existing planes or the check schedules,
changes of operation procedures for flight crews and/or controllers.

But as Daniele Procida stated: knowing all of this may not work against flight phobia...

What intimidate me is that the percentage of accident is low like 0.001%, but when it happens, the percentage of losing all passengers their lives is 100%. No accidents had survivors.

There are accidents that have survivors.  For example, American Airlines flight 383 in October 2016.  One of the engines caught fire (this pretty bad), but there were 0 fatalities.  A few had minor injuries but that's it. There are many other recent example of crashes that did not have 100% fatalities.  It's just that the accidents that have 100% fatalities get publicized a lot more, so you hear about that, but you don't hear about the others.  "Plane crashes, everyone is fine" doesn't make a great newspaper headline

@droidnation Don't ever base your statistics on how you perceive things in the news. By that logic, you can't visit banks because they're only in the news when they're robbed, you can't buy phones because they're only in the news when they break, and you should never even go abroad because every time foreign countries are in the news, something bad happens there.

@Sanchises "...and you should never even go abroad because every time foreign countries are in the news, something bad happens there." Heck, every time your own country is in the news, it's typically because something bad has happened! At best, when it's in the news, that's because something has happened which, for all but a (relatively) small group of people, what's happened is a relatively neutral event. Remember the definition of news: stuff that almost never happens.

@DanielKiracofe: ""Plane crashes, everyone is fine" doesn't make a great newspaper headline": I disagree: The "Hudson Miracle" made very good headlines although it was just that.

@droidnation you can skim through this database:  http://aviation-safety.net/database/ You'll see that there are lots of entries with number of killed persons being 0.

The answer to why it is called the safest form of travel is pretty straight forward.
Automobiles resulted in 45 deaths per BILLION passenger miles.
Trains resulted in 29 deaths per BILLION passenger miles.
Air travel resulted in 6.9 deaths per BILLION passenger miles, .07 per BILLION passenger miles if you're just looking at commercial air travel.
Every time you read, see or think about an airliner accident.. try to remember that for every one of those accidents there are 3-4 MILLION flights that landed with no problems.


text:
Can asymmetric thrust occur for Single engine airplane at idle power during stall practice?
As stated in the title: why can asymmetric thrust occur for Single-propeller-engine airplane at idle-power during stall practice?

It's not asymmetric thrust (an impossibility with one center placed engine), it's the air from the propeller hitting the tail combined with p-factor. It's much more pronounced at power on stalls.

@RonBeyer Looks like an answer. :)

The phenomenon you speak of is called P-factor.  It is the result of a non uniform angle of attack for the propeller blades throughout the propeller disk.
A typical piston powered single engine aircraft has a propeller which turns in a clockwise direction as viewed from the cockpit.  In this fashion the propeller blades are ascending to the left of the pilot and descending to the right of the pilot.  During straight and level cruise (high speed) flight where the airplane’s angle of attack (AoA) is minimal the propeller blades on the ascending and descending sides of the propeller disk have approximately the same angle of attack as they rotate causing the thrust force to be uniform over the entire area of the propeller disk.  But as the airplane enters a higher AoA either when maneuvering or maintaining altitude during slow flight, the angle of attack for the propeller blades on the descending side of the propeller disk have a greater angle of attack than do the blades on the ascending side of the disk.  This causes the blades on the descending side to generate more thrust for a given engine speed which results on an uneven thrust loading over the entire propeller disk, resulting in a counter clockwise torque about the airplane’s vertical axis.  P-factor is most pronounced at high engine power settings at the critical AoA for the airplane where the thrust asymmetry is most acute.
P-factor plays an important part during multiengine operations as well.  When an airplane had two or more wing mounted engines all turning in the same direction, there is always one engine such that, if it should fail, the yawing moment generated by the P-factor of the remaining running engine will be the greatest as compared to if the engine which is currently running had failed instead.  This engine is called the critical engine and creates the most difficulty in maintaining directional control should it fail.

If attitude is strongly nose-high, the propeller push of a single-engine plane may have a significant asymmetry between the left-half and the right-half of the disk. It may have some yawing effect, but that can't be important at idle power...

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
Can any aircraft do donuts under their own power without significant damage?
I was reading this question and trying to think of ways to park an aircraft tail-to-terminal without using reverse thrust, and this question popped into my head:
Are there any aircraft that could, under their own power, do donuts on the ground without risking significant damage to the aircraft?
Intuitively, it seems like there'd be two main problems:

Donuts are hard on the tyres even for automobiles, and for aircraft, which usually use much-higher-pressure tyres carrying much-higher loads than automobiles, dragging the tyres sideways across the tarmac could be expected to produce a much-higher risk of serious tyre damage than with an automobile.
Keeping the aircraft from accelerating forwards during the maneuver would probably be difficult.

@Bianfable: Under their own power.  Question edited.

It is called ground loop (https://en.wikipedia.org/wiki/Ground_loop_(aviation) ) isn't it? :-) Maybe not exactly, but it shows you main weak point, fuselage, landing gear attachment etc is generally rather weak against such forces.

Certainly many small planes can turn with one main not moving (pivot around the wheel). No great smoke show though.

A tailwheel airplane on ice can do donuts nicely for multiple revolutions without dragging a wingtip.  Wheels sliding very sideways but admittedly not smoking! At least, that's been my firsthand experience with radio-controlled models.

@MichaelHall: Right, but that doesn't (necessarily) keep you from drifting an aircraft sideways via other methods (for instance, the one outlined in this comment).  Hence the question.

@MichaelHall: I asked the question due to the possibility of there being methods of drifting an aircraft sideways; that comment is a partial example of what I was looking for (only partial because it's a comment rather than an answer and doesn't mention if any aircraft can actually do that in practice).

Here's a video of a bush plane doing donuts.

@AdityaSharma, merely “going around in circles” isn’t the same thing as doing a donut… A donut entails breaking traction on the back tires(s) to make a circle much tighter that otherwise possible, like this: https://www.youtube.com/watch?v=PY6Xh5YyfYQ That bush plane is actually doing the opposite, making circles much larger that it is normally capable of turning.

@MichaelHall Well then that's probably the closest we can get to a plane doing donuts  because as you said, in order to brake traction at the rear wheels, you need to transmit power through them (or in FWD cars, you must use the handbrake to lock the rear wheels), and planes normally cannot do that. I do agree with what you say about the turn radius...

@Vikki Attempted donuts on a C172 in my simulator; the nose wheel lost traction well before the rear wheels. My theory is that 1) only a small fraction of the weight is on the nose wheel; 2) There is only one nose wheel, (and 2 main wheels); 3) The nose wheel has lateral load on it (since it's being used for directional control), and that encourages an early skid (similar to understeer in a car). So it doesn't look like doing donuts in a tricycle gear aircraft is possible (unless the rear tyres have very low friction coefficient, or the CG is unusually forward).

No.
Burnouts, fishtailing, powerslides, drifting and donuts are done by breaking the traction of the back tires on high horsepower rear wheel drive cars.  Once the wheels are spinning there is then very little resistance to side forces.  Aircraft do not power their wheels, therefore there is no similar means to break the back end loose.
The only way to enter a slide would be to make a hard turn at high speed.  However, if you tried to do this on anything but the most slippery icy surface the aircraft would likely just tip over and scrape the outside wing.
Other that very low CG race cars, most cars will roll if thrown into a sideways skid on dry pavement.  Aircraft typically have an even higher CG than cars, plus a narrower track and taller struts, so there's no good reason to expect you could perform a controlled slide.
But, really there's no need to even consider such an option since airplanes can turn much tighter than cars by locking the inside wheel and pivoting around it.

I don't know much about "drifting" but I thought it's what the "cool kids" were into and they seem to like front-wheel-drive imports --?? -- just a thought-- I really don't know anything about it, occifer Friendly (joke) -- may or may not be related to a way to edit answer to improve--

@quietflyer, No, pretty sure drifting is the exclusive domain of RWD cars.  One of the "cool kids" works at my place, and he explained how he modified his AWD Subaru to RWD only for drifting by disconnecting the front transfer case.

Cool-- got it--

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How controllable is the 737 in single-engine manual-reversion flight? Is it controllably landable (or go-around-able) in this condition?
The 737 is practically the only civilian jetliner still in production1 to have provisions for flight-control manual reversion; in the event of a total failure of the A and B hydraulic systems, the 737’s elevators and ailerons are controlled by aerodynamic forces generated by servo tabs mechanically connected to the pilots’ control yokes (the input to these tabs is locked if hydraulic pressure is available in the A or B system; the locking mechanism requires hydraulic pressure, and disengages if pressure is lost in both systems, allowing the control tabs to be used to control the elevators and ailerons).
The 737’s rudder is a rather different beast.  It has no control tabs, and has no significant manual-reversion capability;2 on the other hand, in addition to the A and B hydraulic systems, the rudder is connected to the standby hydraulic system, which, although not normally pressurised, can be manually activated by the pilots if necessary (and automatically engages in the event of an A or B hydraulic system failure during critical portions of takeoff or landing).  Nevertheless, a failure severe enough to disable both the A and B hydraulic systems (such as an uncontained engine failure, major wing fire, or rudder separation) would have a good chance of also taking out the standby hydraulic system, which powers several of the same components as the other two hydraulic systems (namely, the rudder, thrust reversers, and leading-edge high-lift devices), and, thus, would be vulnerable to being disabled by a failure that also ruptured the A and/or B hydraulic lines serving those same components.
Even in the event of a total hydraulic failure, though, the elevators’ and ailerons’ manual-reversion capability would still keep the aircraft perfectly (if tiringly) flyable, since the rudder stands idle in normal flight, being used in the air only for countering the large yawing moments produced by an engine failure (especially at low airspeed)7, and engine failures are very rare nowadays...
...except that a destructive engine failure, combined with the failure of the engine cowling to contain the resulting debris,8 could easily be the cause of a multiple-hydraulic-system failure.  For instance, an uncontained turbine rotor burst in the #1 engine would almost certainly rupture at least some part of the A system’s engine-driven hydraulic pump, would likely breach both the A and standby lines in the #1 thrust reverser, and could easily penetrate the B and standby lines powering the left wing’s leading-edge high-lift devices, leaving the aircraft with one engine and no hydraulics.9
In single-engine manual-reversion flight, the only way possible to counter the yawing moment produced by the asymmetric-thrust condition is to bank the aircraft towards the operative engine; the positive bank/yaw coupling12 resulting from the 737’s rear-mounted vertical tail produces a yawing moment in the direction of the bank, countering the yawing moment from the thrust asymmetry.  This would likely require a fairly large bank angle13 to produce a sufficiently-powerful restorative yawing moment, and the ailerons would have to maintain this bank without the help of the spoilerons (which are inoperative in manual-reversion flight) and against the considerable rolling moment created by the asymmetric-thrust-induced sideslip angle and the positive slip/roll coupling14 resulting, in part, from the 737’s rearward-swept wings.
Even if the aircraft were controllable in manual-reversion flight with one engine out at high airspeed, it might not be controllable at lower airspeeds (such as one would normally use for landing), as the lateral (roll) control authority of ailerons decreases considerably at low airspeeds due to the higher angles of attack required to maintain flight at a slower airspeed (this is because a given aileron deflection increases the downgoing aileron’s lift coefficient less and less as the AoA of the wing as a whole - and, thus, the starting AoA of the aileron - increases towards the stall angle, causing the ailerons to produce a smaller and smaller proverse rolling moment as the wing’s attack angle increases, while the increase in the downgoing aileron’s drag coefficient for a given aileron deflection gets larger and larger as the wing’s AoA increases, causing the ailerons to produce a larger and larger adverse yawing moment at high attack angles15).16  This would be further aggravated by the unavailability, in manual-reversion flight, of the aircraft’s spoilerons and trailing-edge flaps - the former would assist the ailerons in rolling the aircraft, being especially useful at low speeds (as they are immune to the severe degradation in control authority experienced by ailerons at high attack angles, producing proverse rather than adverse yaw, in addition to proverse roll17), while the latter would increase the wing’s lift coefficient for a given angle of attack, allowing it to support the same weight at a lower AoA, shifting the wing into a more aileron-authority-friendly regime.
This would seem to require the aircraft to attempt a landing at very high speed while maintaining a fairly steep bank angle until just before touchdown, and I don’t see how it would be possible to go around without the increases in engine thrust and vehicle angle of attack causing a loss of lateral/directional control, or to avoid a high-speed excursion from the side of the runway immediately following touchdown.
The controllability issues in single-engine manual-reversion flight would presumably vary between different 737 models.  All else being equal, one would expect that the variants with lower engine thrust would be easier to control than those with more powerful engines, due to the lesser thrust differential with one engine out (so an Original would be more controllable in this state than a Classic, which would itself be more controllable than an NG, which would still be more controllable than a MAX); that variants with higher wing loadings would be harder to control, due to their need to fly at higher attack angles (and, thus, in less-aileron-friendly regimes of flight); and that longer-bodied variants would be easier to control, due to the stronger bank-yaw coupling resulting from the vertical tail’s longer moment arm.
How controllable are the various 737s, actually, in manual-reversion flight with one engine out?  Are any of them easily (or at all!) controllable attempting a landing or go-around in this condition?  Are there other differences between the variants in this regard that I missed?

1: At least, if we overlook the (update: no-longer-) current pause in 737 production.
2: If you want to get really pedantically technical, a small amount of rudder deflection is attainable even without any hydraulic pressure, due to the design of the 737’s rudder system3; however, this requires the application of an extreme amount of force to the rudder pedals (approximately 300 pounds4 per inch of rudder pedal deflection after the first inch, in addition to the additional force required to push the rudder pedals the initial inch required to take up slack in the rudder control system), and the small amount of rudder deflection available isn’t enough to generally be useful.
3: The gory details, straight from one of the metaphorical horses’ mouths:

During normal operation of the rudder in flight, if a pilot applied between 9 and 70 pounds of force to a rudder pedal, the rudder would move in response until it reached its blowdown limit (when the aerodynamic forces acting on the rudder surface equal the hydraulic actuator force). According to Boeing engineers, if the pilot were to then apply additional force to the rudder pedal, the pedal would move about 1 inch farther, with no corresponding movement of the rudder, as the slack in the rudder linkage system is removed and the external input crank contacts the external stop. Any additional pilot application of force to the rudder pedal would result in rudder pedal movement of about 1 inch for each 300 pounds of rudder pedal force, which in turn would move the rudder surface slightly beyond the maximum deflection possible from the hydraulic actuator force.  [NTSB AAR-99/01, page 26 (page 50 of the PDF file of the report), and NTSB AAR-01/01, pages 16-17 (34-35); the exact same text is present in both (these two AARs deal with a pair of very similar accidents).  Although the quoted text describes the response of the rudder system to a sufficiently-forceful pedal input when the rudder is under full hydraulic power and at its aerodynamic blowdown limit, the same mechanism is applicable when the rudder is completely without hydraulics; the only difference is that the small directly-pilot-induced rudder deflection is relative to the rudder’s neutral position rather than its blowdown limit.5]

4: For reference, the maximum force that an average pilot is physically capable of applying to an aircraft’s rudder pedals is generally around 500 pounds (according to ergonomic studies cited by the NTSB in AAR-99/01 and AAR-01/01).
5: If you want to get really really pedantic, this deflection is still relative to the blowdown limit, even with fully-depressurised hydraulics; it’s just that, with no hydraulic pressure available, the rudder’s blowdown limit is 0° (the neutral position).6
6: Note that the calculation of the rudder’s blowdown limit takes into account only the rudder-deflection force provided by the rudder’s hydraulic actuators - the additional force available directly from the pilots’ feet if they press really hard on the rudder pedals (and the only force available for moving the rudder in the event of a total hydraulic failure) is not accounted for.
7: The rudder has three additional major functions in large aircraft (aligning the aircraft with the runway immediately following touchdown during a crosswind landing, to keep the tyres from being destroyed by the crab angle - sometimes quite large - that needs to be held until touchdown to keep the aircraft from being blown downwind of the runway; keeping the aircraft on the runway during a crosswind takeoff; and taking evasive action to avoid a collision at high speed during takeoff or landing roll), but these are purely ground phenomena.
8: Virtually guaranteed if the destructive engine failure takes the form of a full fan, compressor, or turbine rotor burst (due to the extreme amounts of kinetic energy released in these types of failures); less common for a simple blade-out (although even these can sometimes result in uncontained failures).
9: As for the other hydraulically-powered components on a 737, here’s a quick rundown of their statuses with the A and B systems both failed, or with the A and B systems plus the standby system all nonfunctional:

Landing gear: Can be extended by means of gravity, but cannot be retracted.
Nosewheel steering: Inoperative - the nosewheel casters freely.
Wheelbrakes: Powered by the brake accumulator for six full applications or equivalent (which should be enough to get the aircraft stopped on the runway).10
Spoilerons: Inoperative - lateral control is by ailerons alone.
Thrust reversers: Both operative if standby hydraulic system functional; otherwise, both inoperative.
Horizontal-stabiliser trim: Operative using the manual-pitch-trim wheel on the cockpit center console, but this may require an extreme amount of force at higher airspeeds and/or more-adverse longitudinal center-of-mass positions.11
Ground spoilers: Inoperative.
Autopilots: Both inoperative.
Trailing-edge flaps: Inoperative Operative at reduced rate using the backup electrical flap drive system.
Leading-edge high-lift devices: Operative if standby system functional; otherwise inoperative.
Autoslats: Inoperative.
Yaw damper: Inoperative.

10: In theory, one could use differential braking (applying wheelbrakes only, or more strongly, on the side with the operative engine) to at least attempt to maintain directional control after touchdown; however, this would further increase the already-greatly-lengthened (due to the need to maintain a very high airspeed in order to maintain lateral/directional control before touchdown in single-engine manual-reversion flight, plus the inoperability of the spoilers and thrust reversers in the event of a total hydraulic failure) rollout distance needed, and would risk depleting the stored brake-accumulator pressure (as keeping the aircraft on the runway throughout rollout using differential braking alone would likely require at least a few changes in braking force, or even momentary brake reversals, which would quickly eat through the six full applications' worth of pressure in the accumulator).
11: The potential need to apply a very great deal of force to move the manual-pitch-trim wheel is especially severe on the 737 NG and MAX, which have a smaller manual-pitch-trim wheel (and, thus, provide the pilots with less of a mechanical advantage when trying to turn said wheel) than the 737 Original and Classic; for the NG and MAX at high airspeeds and/or adverse CoM locations, it may be physically impossible to trim the aircraft in pitch without the electrohydraulic trim system.
12: I.e., banking the aircraft to the right makes it want to yaw to the right, and vice versa.
13: Yes, I am aware that all non-centerline-thrust multiengine airplanes are required (at least in the U.S.) to demonstrate the ability to maintain lateral/directional control with one engine out without requiring a bank angle of greater than 15° away from the failed engine; however, this is with the rudder producing its own yawing moment to assist (quite potently) that produced by the aileron-induced bank.  With the rudder down for the count, and the entire burden of resisting the asymmetric-thrust-induced yawing moment falling on the aircraft’s bank angle and bank/yaw coupling, the bank angle necessary to maintain lateral/directional control would be considerably greater.
14: I.e., placing the aircraft in a nose-right sideslip makes it want to roll to the right, and vice versa.
15: This is why aircraft have a crossover airspeed - a speed below which the rolling and yawing moment from a full-scale rudder deflection cannot be countered using the aircraft’s lateral controls alone.
16: More advanced aileron designs can reduce or even eliminate adverse yaw, mostly by generating enough extra drag on the upgoing aileron to balance out the extra lift-induced drag on the downgoing aileron (such as the differential aileron, which deflects the upgoing aileron much more than the downgoing aileron, and the Frise aileron, which has the lower front edge of the upgoing aileron protrude into the airflow below the wing), but these are less and less effective at higher and higher angles of attack.
17: As a result, one would expect that aircraft rolled solely by spoilerons and completely lacking traditional ailerons (such as the B-52G/H or the MU-2) would be immune to the crossover-airspeed phenomenon, although my question to that effect has yet to attract a useful answer.

That's a triple failure. I wouldn't be surprised if it's beyond 1e-9 (i.e. no need for consideration).

The A systems are 2 EDPs, and the B systems are two EMDPs, plus the standby. The Y/D runs off the B system and you can have another Y/D running off the STBY.  You do need the rudder in both-engines flight for Y/D function to counteract adverse yaw and to suppress dutch roll.  But in  any case, an uncontained engine failure is only going to take out that side's A plumbing.  There is enough redundancy to exceed 1e-9 for any single point failure I would say which is what the cert requirements are.  Generally, double simultaneous failures need not be accounted for in risk analysis.

@JohnK We do need to consider multiple failures, as long as the combined probability is above 1e-9, as per 25.1309. However, in this case, I still feel that the combined would be below the extremely improbable threshold.

Footnotes with footnotes? It's gotta be a Sean question! Some of the most researched questions I've seen on any stack!

@JohnK: No, each of the A and B systems runs to systems on both wings, allowing a single failure to take out both systems - and the standby system, for that matter, which also runs to systems on both wings.

@JZYL: Figuring out the probability of multiple simultaneous failures by multiplying the probabilities of each separate isolated failure only works if the failures are all independent of each other - which, in the possible scenarios mentioned in the question, they aren't (as one failure is the direct cause of the others).

@Sean On the wing and elevators, each hydraulic system feeds to a separate PCU; so a single point failure cannot result in loss of both systems. I'm not sure about the rudder. But any recent (at least 1980s onward) certified aircraft must cater to rotor burst, such that a single fragment of unlimited energy should not result in catastrophic failure(s). Losing an engine due to rotor burst and knocking out both hydraulic systems sure sound like a scenario that should've been considered.

@JZYL: The hydraulic systems do indeed feed to separate PCUs now (they didn't used to, which caused a few issues).  I wasn't thinking of PCU damage anyway; I was thinking of engine fragments breaching the systems' hydraulic lines within the wing.

@Sean I don't know the 737 that well, but if a turbine burst can take out A and B systems at the same time, that is a mind blowingly bad design. Do the B system lines run right alongside the engine?

@JohnK: All three hydraulic systems (the A, B, and standby systems) service components either directly attached to the engines and surrounding the turbines (thrust reversers) or near them and in line with the plane of a bursting turbine rotor (leading-edge high-lift devices), and, thus, can presumably be breached by a single turbine rotor burst.

I don't understand the part (footnote 10) about keeping the aircraft on the centerline after touchdown. At idle, the operating engine would create negligible thrust, and if that is a problem, the pilots can shut it down. I'm not sure how much rudder/nosewheel control is required to keep the plane on the runway due to other effects like crosswind or runway/braking irregularities.

@EfeBallı: A high thrust setting on the operating engine would be necessary up until touchdown just to maintain flight, and jet engines spool down (and up) very slowly.

With TE flaps INOP, that would be a rather fast and low drag final approach, I don't believe the final approach thrust setting would be much higher than idle.

@EfeBallı Are the TE flaps going to be INOP? Footnote #9 says they have an electric backup, so I'd assume the surviving engine can power that.

I read that too, I missed it or the footnote was edited after my comment.

@EfeBallı: The latter.  Sorry 'bout that - I only recently found out that the 737's TE flaps can be operated electrically.

No probs. As a self-taught simulator pilot, I'd still be reluctant to dump full flaps on a single engine 737 that I fly on muscle power.

@EfeBallı: Dropping flaps would increase the aircraft's drag, requiring higher thrust on the operative engine to counter, increasing the thrust asymmetry, and, therefore, requiring more lateral-control input to counter... but would also decrease the AoA of the rest of the wing, increasing the lateral-control authority available for countering the thrust asymmetry.  As I'm not a 737 pilot, I don't know off the top of my head whether the combination of these two opposing effects would make lateral control easier or harder to maintain.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why are the 737's variable-speed generator drives so unreliable?
The 737 Classic has two different possible styles of engine-driven generators, distinguished by the generator's drive mechanism: the constant-speed drive (CSD) and the variable-speed constant-frequency drive (VSCF).
The original VSCF drives were very unreliable, failing roughly three times as often as the constant-speed drives.  A 2001 modification programme by Boeing and the generator manufacturer improved this somewhat, but the generator drives remained unreliable enough for VSCF-equipped 737s, even those with modified drive units, to be subject to stricter limitations than other 737s (the United Kingdom's CAA requires them to remain within 45 minutes' flight time of a suitable airport, even if only one of the generators is VSCF-driven and the other has the reliable constant-speed drive; I don't have information on whether other regulatory authorities have similar limitations for VSCF-equipped 737s).
Why are the 737's variable-speed constant-frequency generator drives so much less reliable than the same aircraft's constant-speed drives?

No idea how it works, but VSCF, just from its name, sounds like a mechanical monstrosity.  Usually what happens is the maker, HS in this case, will tweak this and tweak that trying to get the MTBF up without spending too much money, when the unit in reality has some fundamental weakness in its architecture that will cost bajillions to fix.  So they spend years dancing around the problem until something new comes along to replace it.

By drive do you mean mechanical parts of the system? Constant speed system is way more complicated than a variable speed one, so I'd say this is likely to be a case of shoddy electronics. Caveat: I'm not a mechnic or an electrician...


text:
Do any aircraft use throttle steering during normal flight?
If the engine(s) on one side of a multiengine airplane are set to a higher throttle setting than those on the other side, the airplane (all else being equal) will yaw towards the lower-throttle side; this is best known as a technique for controlling airplanes that have suffered a partial or complete failure of their primary flight control system(s).
Are there any airplanes that make use of throttle steering during normal operation?

I doubt any aircraft uses this as a normal method of control since it really is a poor way of doing things. Asymmetric thrust doesn't just affect yaw, one wing will drop and is difficult to get level again if it degrades too much. It also becomes a problem if one of the engines happen to fail.

I take it my small cheap styrofoam dual channel RC aircraft isn’t what you’re after, although it technically qualifies, the way the question is worded.

Aircraft do not steer by yawing, but by banking. The rudder is needed to compensate for the adverse yaw of entering the bank, but in a steady turn on most aircraft it is deflected very little. Differential thrust would not help anything.

Nearly all quadcopters! (“drones”)

@Sean, please clarify your question, do you mean aircraft as in anything that takes to the skies, or airplanes, which is much more specific. Also, do you mean manned or unmanned craft.

Define "normal operation." The question is rather vague.

Yes, while rolling on the ground. Once in the air: No. It's the other way around: With unsymmetrical thrust you should avoid certain maneuvering (never roll into the dead engine and such).

Yes, The Northrop Grumman B-2 Spirit Stealth Bomber.

The leading edge of the wing has an internal structure that helps it
  absorb radar energy. The outermost wing segment features a "rudderon"
  or "deceleron", a vertically-split airbrake / rudder that
  simultaneously opens up and down. To act as an airbrake, both the
  decelerons are opened, while to act as a rudder only one is. This
  clever gimmick goes back to the original Northrop flying wings. There
  is an elevon inboard of the deceleron on the outermost segment of each
  wing, and then two elevons further inboard, on the next segment.
  Finally, there is a single control surface for pitch control on the
  "beavertail" at the center end of the aircraft, giving a total of nine
  control surfaces.
The decelerons have to be opened about five degrees before they are
  effective, and in normal cruising flight they are left slightly open.
  However, this undermines stealth, so when the bomber is in hostile
  airspace, it uses differential engine thrust for yaw control.

"Yaw control" would normally just mean keeping the airplane aligned with the airflow.  If we are talking about turning, then we are talking about banking as Jan Hudec mentioned.  So is the B-2 using engine induced yaw to skid the plane, thereby inducing a bank, and getting a turn that way?  Or are the elevons inboard still doing the job of banking the plane and it's just differential thrust being used keep the turn coordinated?  I would tend to think it's the latter.

This does not qualify as throttle steering because yaw is not about steering, it's about compensating for other forces during steering,

Another aircraft is the AeroVironment Helios, which was solar powered and flew with ten or fourteen electric motors. The torque of an electric motor can be controlled very rapidly and precisely, far faster than gas turbines.

To turn the aircraft in flight, yaw control is applied by applying differential power on the motors — speeding up the motors on one outer wing panel while slowing down motors on the other outer panel.

In fact, in this flying wing design, differential thrust also provides pitch control:

A major test during the initial flight series was the evaluation of differential motor power as a means of pitch control. During normal cruise the outer wing panels of Helios are arched upward and give the aircraft the shape of a shallow crescent when viewed from the front or rear. This configuration places the motors on the outer wing panels higher than the motors on the center panels. Speeding up the outer-panel motors caused the aircraft to pitch down and begin a descent. Conversely, applying additional power to the motors in the center panels caused Helios to pitch up and begin climbing.

Had the aircraft not crashed, the eventual plan was to remove the elevators, the only control surface, and fly completely by differential thrust.

Maybe we should specify that it should be aircraft that successfully used differential power for this. In reality, it was not yaw control that gave them steering, but the fact that a drop in power on one side and an increase in the other would cause one wing to generate more lift than the other, inducing a bank. That is what provided the steering control. But in the end, it was a poorly designed aircraft.


text:
Why is Bombardier selling off all of its airliner series?
Bombardier’s product lineup formerly had three separate families of airliners (in addition to its side business with the Challenger, Learjet, and Global business-jet series):

The DHC-8 (Dash 8, Q-Series1) line of 39-to-90-seat regional turboprops, acquired with Bombardier’s 1992 assimilation of de Havilland Canada (hence the “DHC” in the name); the DHC-8 itself had originally entered service back in 1984.
The CRJ (Canadair Regional Jet) line of (surprise!) regional jets, which entered service later in 1992 as a stretched airliner version of the Challenger 600 business jet (which was originally produced by Canadair, and was acquired, along with its parent company, by Bombardier in 1987, thus explaining the “Canadair” in the CRJs’ long-form name); the original version was the 40-to-50-seat CRJ100/200 series, produced until 2006, but this was later superseded by the 50-to-104-seat CRJ700 series, which entered service in 2001.
The CSeries line of 108-to-160-seat midrange jets, a homegrown Bombardier design which entered service in late 2016.

Recently, however, Bombardier has been selling off their airliner lines seemingly as quickly as they can find buyers to take them off their hands:

In mid-2018, Bombardier sold a majority stake in the CSeries to Airbus (who renamed it the Airbus A220), and sold off its remaining stake in the project in February 20202 (although Bombardier does continue to build them at its Montréal factory, albeit merely as an Airbus contractor).
In mid-2019, the DHC-8 was sold to Longview Aviation Capital, which resurrected the De Havilland Canada name for a company it established to build the aircraft.
Finally, Bombardier is currently (as of mid-2020) finalising a deal to sell the CRJ series to Mitsubishi Heavy Industries, which would drive Bombardier out of the airliner market completely.

These sales are allegedly to improve the company’s profit margin; however, divesting oneself of one’s flagship products seems completely self-defeating (in a boiling-the-seed-potatoes kind of way) as a strategy for making more money, as it eliminates their ability to sell more of these aircraft in the future.  (Moving out of the airliner business to focus solely on business jets is looking especially stupid right now, given that business aviation, unlike commercial aviation, is unlikely to recover more than partially from the current state of affairs.)
What is Bombardier trying to get at here?

1: Technically, the Q-Series moniker only refers to the later DHC-8s equipped with active-noise-suppression equipment; however, with the exception of the DHC-8-100 (which was never equipped with the system, and which left production in 2005), all DHC-8s delivered since mid-1996 are so equipped, including all DHC-8-400s.
2: Thanx to @JZYL for pointing this out.

Bombardier is [selling the rest of CSeries stake] (https://www.ctvnews.ca/business/bombardier-is-out-of-commercial-aviation-with-plan-to-sell-c-series-stake-1.4809896) as of Feb 2020. In survival business, cash is king.

You mention the virus chaos as an indication that focusing on business jets is a bad decision. I'd wager that 99.99% of the decisions to go this direction were made well before the Wuhan virus became an issue.

@FreeMan: I am aware of that; I mentioned it as more of an "in hindsight" thing.

It's important to understand that Bombardier Transportation/Aerospace, while ostensibly a company separate from the original Bombardier family (which only retained a minority stake when it was split off from Consumer Products - snowmobiles, watercraft, etc), is still actually controlled by the family due to a bizarre dual class share structure where the family retains its minority stake in the form of class A shares that have many more votes per share (I think it's 9 or 10 votes or something like that) than the single-vote class B common stock.  This gives the family control of the company, and makes it takeover proof (also making it very difficult to raise private capital), even though its equity stake is a small minority one.
But the basic problem is the C series and to a lesser extent the Global 7000, just about broke the company.  So broke they were forced to give the program to Airbus more or less for free.  The CRJ line was neglected due to the commitments for the C with little in the way of proper design updates.  This handed over dominance on the Regional side to Embraer (who's newest Ejet is basically a baby C-series technology wise) and CRJ production is winding down (it is now a Mitubishi product - M's interest wasn't in the Regional Aircraft program, but its support and spares organization, where M was hopelessly lacking).
On the corporate side, the Lear 85 disaster and the Global 7000/7500 development was another money hole, almost as bad as the C.
So in nutshell, since the traditional government money tap the company has traditionally relied on has largely dried up, the family has decided to dump everything (including rail) and just retain the consumer products operation, the separate family controlled company, upon which the whole business was founded, that was spun off some years ago.

The reason they are selling aircraft manufacturing lines to improve profit margin is because their aircraft business has a relatively low-profit margin. One of Bombardier most successful products is this:

Bombardier Zefiro. Source: Bombardier
It looks somewhat like an aircraft but it is missing the wings. It is called a "train".
Bombardier calls itself "the world's leading manufacturer of both planes and trains". Compared to Airbus and Boeing, they are a very small aircraft manufacturer, but they compensate for that in the train market. By moving out of the airliner manufacturing market they focus on their more profitable train building business.

Actually, they have a tentative agreement to sell the train division to Alstom, before COVID.

The train business doesn't look like it will last long. The world largest market China has licensed the production and shifted to demostic production

@user3528438: That might be an American perspective. Europe is definitely looking at trains to replace planes. E.g. Air France has to drop a number of domestic routes in favor of trains, as a condition to their bailout. China definitely is a growth market, but that's all additional demand.


text:
How are procedures for curved-runway operations different from those for straight runways?
Runways having a significant curve between the two ends aren't exactly common, but they do exist, one good example being Elk City Airport's runway 14/35.
When operating to or from an airport with a curved runway, what does one have to do differently than with a straight runway?

Hi. Could I ask that you use your privilege of suggesting new tags carefully. before adding new tags please consider if they might have wider usage, or whether they're just a bit too niche.  A good indication (although not a perfect one!) is that there would be a fair amount of existing questions that would benefit from the tag. In this case [curved-runways] was probably a bit niche for now.

I know that KAVX has quite a hump in the middle of the runway which does not allow for departing aircraft to see all the way down the runway, and I'd imagine that the procedures are the same as they would be for Elk City Airport. The Facilities Directory/Chart Supplement for KAVX says:

PILOTS CANNOT SEE ACFT ON OPPOSITE ENDS OF RWY DUE TO GRADIENT, MUST ANNC TAKING ACT RWY ON UNICOM PRIOR TO DEP

Essentially, communicate your intentions on the common traffic frequency. Where you are taxiing, what runway you are using and when you're about to take off. Also be vigilant watching for traffic that may be using the opposing runway.
On a curved runway, Your main concern is that someone else would be approaching or using the opposite runway while you are using the other end. This is even recommended in the Chart Supplement for Elk City, which says

LAND RY 14 TKOF RY 35 WHEN WIND CONDITIONS ALLOW

As far as actually taking off on a runway that is not straight, you'd need to take into consideration obstacles around the runway environment. In the case of Elk City, I'd hold off on rotating until after the bend so that I can climb straight out and get above tree level prior to making a turn.  It appears as though that is in line with the Chart Supplement instructions, as landing RWY14 would give you the longest length to land and start to slow before you had to make the turn, and taking off on RWY35 would have you turning prior to rotation speed (or at least shortly thereafter).

If anyone is curious, here's a video of a Cessna landing on Runway 14 at Elk City.  Watching that video makes me nervous, but then I'm not a pilot.

I'm wondering why that made you nervous, to me it actually makes the curve in the runway look pretty benign. Seems like the pilot was plenty slow and under control and it hardly seemed like much of a turn while he slowed down. Is it the unimproved (grass) strip that makes you nervous?

I think my nervousness stems from the fact that I don't really have a feel for how much steering control a plane has if it's moving at more than "automobile speed" on the ground.  The grass strip also made it seem less in control.  But I've never even flown in a Cessna, let alone piloted one.

A small Cessna moving at more than automobile speed will be flying.


text:
What’s the difference between Arrival and Approach controllers?
Some airports have two separated frequencies related to arrival.
Arrival and approach.
When I navigate to some airports that have both frequencies, I first contact the arrival controller  then, at a specific point, arrival controller hands over me to the approach controller, who has a discrete frequency.
What's the difference between the role of arrival controller and approach controller?
Conversely, when I navigate to some airports that only have an approach frequency, approach controller controls me until contacting tower controller.

757's answer is pretty comprehensive. But to clarify a few things...
In the US, the TRACON—Terminal Radar Approach CONtrol—consists of one-to-several sectors. Like 757 said, these sectors have different names internally; these names might refer to functions (Arrival, Final, Departure, Satellite), relative positions (High North, Low East), or an important fix within each sector (more common at ARTCCs but it shows up at some TRACONs too).
If sectors are named according to function, that is only a generality; each controller is responsible for providing required separation and other services just the same as all the other controllers. It might be the case that an "arrival" sector works mostly arrival streams, and "departure" works mainly departing aircraft—but that's a post-facto definition. All of the controllers will use the callsign "Podunk Approach" when talking to pilots, except if the pilot has just departed an airport in which case the controller will probably identify themselves as "Podunk Departure." But when getting a frequency change to another sector, or when a neighboring facility gives you a frequency change to the TRACON, they will always reference "Podunk Approach"—even if you happen to be going to the sector named "Departure."1
A "Final" sector will more likely work nothing but final traffic, but this is still not to say that they can't talk to an overflight or something like that.
There is one exception. When conducting radar approaches, that is, an instrument approach where the guidance is not from a NAVAID or GPS but instead from a controller issuing very precise vectors, the controller will identify themselves as "Podunk Final Controller," and that's also what the controller before them will say when switching you. This controller's sole job is to vector you down the final approach course to the runway, and other controllers in the TRACON should keep their traffic away from you without bothering the final controller.

1At my facility one of the neighboring TRACONs has a "Departure" sector which works the higher half of the airspace along our border. I transfer arrivals overflights to them all the time, and instruct the pilots to contact "CityName Approach."

Thanks for the detailed answer.
However, our country adapted ICAO standards.
There's no definition of TRACON.. ; (
In the wide range, is it true why the airport authorities separated arrival and approach only for reducing their workload concentration by having two discrete frequencies and controllers?

Yes, it would be to reduce the workload of the controllers, primarily by reducing the size of the airspace they are responsible for and secondarily by attempting to ensure that all aircraft within their jurisdiction are doing similar things (which is not always possible to ensure). This sectorization is something internal to the air traffic control provider and would be determined by that organization, not by anyone associated with the airport itself.


text:
Why does the EGT gauge in 172s have no numbers on it?
The EGT gauge on the 172 (and other Cessna singles?) doesn't have a numerical scale on it, just markings every 25 degrees.  I know that one is supposed to use the EGT for leaning operations (ROP, LOP, what have you), but it's always stymied me that there are no actual numbers on it.  Any reason why this is so?

The simple EGT gauges do not have numerical scales because you never lean to a temperature.  You find the peak temperature, set the movable red needle to that temperature, and then lean to x degrees lean (or enrichen to x degrees) of peak temperature.
Set the peak temperature at full throttle, full rich at a sea level airport.
This system eliminates the concerns of calibration: Is the gauge calibrated, did the mechanic install the probe in exactly the right spot, etc. etc.

I'm not sure about Cessnas, but on my plane the EGT uses 2 probes - one in the exhaust, and one in the engine compartment.  The gauge simply shows the difference between the two.

Dan P., I'd like more information on your second temperature probe.  Is it measuring ambient temperature in the engine compartment? Something else?  By the way, some planes have a carb temperature gauge, and this second probe might be the source for that gauge.  The PA-28s that I fly have only one probe, into the exhaust pipe.

yep, it's measuring "ambient".  Now that I think about it, it's not actually in the engine compartment, but behind it on top of the fuel tank.  Sorry for the misstatement.

The very first EGT gauges did not have absolute values marked. They only had marked graduations spaced 25° apart. Absolute values of EGT are not particularly interesting for a number of reasons. The most important is that "indicated EGT" is not a "real" temperature. This is because the sensing probe sees nothing for about 2/3 of the time, during most of the intake, compression, and power strokes because the exhaust valve is closed, thus no exhaust gas is flowing out of the exhaust port and past the probe. During the one third of the time that the exhaust valve is open, it would see a constantly changing gas temperature that starts out very hot when the valve first opens, but cools very rapidly as the hot compressed gas escapes and expands, and then ultimately is scavenged by cold induction air during the valve overlap period.
Of course all of this is happening about 20 times per second, and the probe cannot possibly keep up with them. The probe ends up stabilizing at some temperature between the hottest and coolest gas temperature it sees, and dutifully reports this rather arbitrary temperature to the instrument. The temperature reported is not "exhaust gas" temperature, but rather exhaust PROBE temperature, which is stable but related to actual exhaust gas temperature in roughly the same fashion as mean sea level is to high tide.
What IS useful, however, is knowing where on the mixture adjustment that EGT reaches its peak temperature. Thus you lean the engine while watching the EGT indication until it reaches its peak reading. This is the point where the mixture is stoichiometric, where all of the fuel and air mixture is consumed. There is no remaining fuel to cool the mixture, nor remaining air to cool the mixture, thus the EGT indication will be at its greatest at this point.
Once this point has been found, you set the instrument's "bug needle" to this point, then richen the mixture to get an indication that is 50-100°F lower than the bug needle.

If one is looking for a temperature 50-100 degrees lower than peak, wouldn't it make sense to have degrees indicated on the gauge scale? Is there some other unit of measure you meant in that last sentence?

The tick marks are all graduated 20° apart. You still don't need an absolute value to find this point. As I mentioned previously, the absolute value indicated isn't even a valid value. You just lean to a certain number of "ticks" below the peak tick mark.
Besides, this lean method is pointless on carbureted engines anyway due to the imbalance of fuel distribution of a carbureted system.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Does any USAF fighter jet use surfaces near the front for pitch control?
Some European fighters like Eurofighter use frontal surfaces for pitch control. Does any US fighter use this configuration? And what are these surfaces called?

Probably https://en.wikipedia.org/wiki/Canard_(aeronautics) - not used on production USAF fighter jets.

thanks, i was thinking of the Eurofighter Typhoon.

This is an "old continent thing". Typhoon, Rafale, JAS, and Sukhoi use this control method.

These surfaces are called canards.
To my knowledge no canard equipped fighter ever entered service with the armed forces, though there were a few experimental prototypes such as the CCV/AFTI F-16 or the F-15 ACTIVE which made use of canard foreplanes.

The Grumman X-29, Rockwell X-31, and Boeing X-36 are other examples of US technology demonstrators which made use of canards for extreme maneuvering, none of which were ever pursued further for development into a fighter aircraft.


text:
How could a 45 years old DC10 be still in service with one of the largest cargo airline?
It is amazing to see this 45-year-old DC10 freighter still operating for FedEx regularly:

I know the lifespan of an aircraft is not rated by ages but flight cycles, but an actively used DC10 over 45 years must have exceeded the designed flight cycle right?  However, I regularly come across 30+ or even 40+ years old aircraft on FR24 that are still in service, mostly freighter though. Wouldn't they raise any safety concerns? Besides, how could airlines operate them efficiently?

There are actually a few airlines flying DC-3's out there! Buffalo airways out of yellowknife jumps to mind

The 3 has pretty much an indefinite fatigue life.  They get driven out of service mostly by the other two of the Big Cs,  Crashes and Corrosion.

Welcome to aviation.SE! This question is closely related and may be helpful.

The DC-10’s have all been converted from DC-10’s to MD-10’s. The last conversion was in 2007, I believe. The entire aircraft wiring was replaced, the avionics package converted to glass cockpit and other structural work was also performed. As a result, the airframe was “zero houred” upon completion of the conversion.
Although they’re getting older, they’re still warhorses and their reliability is still good.
Also, cargo operators don’t put nearly as many cycles on their aircraft as passenger carriers. As a result, there is a lot less wear and tear on the aircraft.

Not warhorses --> workhorses !

The newest B-52 was built in 1962, so an aircraft's age is not really a factor, as long as it has been properly maintained and it doesn't develop major structural issues. (like cracks in critical structures, like the wing main spar, or one of the main fuse longerons.)


text:
In aircraft with single spar in each wing, are the spars at different chord locations?
In aircraft like the Pipistrel Virus, there's a single spar in each wing which is held with steel pins inside the fuselage.
Are these spars offset along the chord line or are they bent at an angle for assembly?

Single spars are generally located at the thickest part of the wing airfoil (both sides the same); if they disconnect inside the fuselage or cabin for wing removal (as in the video example), they'll either taper, one on the forward side and the other on the aft side, or one will slide inside the other before they are locked together by pins or bolts.

In the video and other picture I found, It seems the combined thickness of tapered section is more than of a single spar. Might they be slightly offset?

It's possible, but it would cost more to make and be inferior to a spar that isn't offset due to stress risers and such -- or heavier to add material offsetting that.

@Mridul Yes, each tapered section needs to be wrapped in shear layers which will combine to more thickness than a single spar. Now consider this: If the spars were at different chord in each wing, each wing would twist differently when bending. You really want to avoid such behavior. Clearly, they must be at the same chord position on each side.

If you look closely at the view from below, you can see that they are built with matching tapers, thin at the end and normal thickness at the root.  The left wing's spar tapers toward the front, and the right tapers toward the back.  When the tapered sections are overlapped together, they form a uniform thickness across.
So the spars in each wing are at the same location chordwise. It's just that they taper across the cabin in different directions so they mate together in a constant thickness beam with a diagonal axis-wise seam, like a scarf joint connecting two wood planks, but without the glue.  Once you are beyond the overlapping middle section, they are the same.


text:
What is the reason that the Russian flying wing UCAV does not have sharp leading edges?

The new Russian UCAV sports a fairly blunt leading edge while all the other UAVs in the same category have sharp leading edges.

This seems like a hint that for this design, flight characteristics are more important than stealth.  Generally, in subsonic flight, anything with sharp leading edges will have poor transitions from negative to positive lift and a sharp, unforgiving stall -- and given appropriate materials, there may be little to gain even from a stealth standpoint with modern design optimizations (the B-2 has much more rounded leading edges than the F-117, for instance).
Since few stealth designs are intended to fly supersonic (shock waves and compression heating are prone to give away your presence, never mind the much higher exhaust temperature that tends to go with supersonic flight), it makes sense to optimize away sharp leading edges if doing so doesn't compromize observability.

will the sharp edges not help in vortex lift?  Is vortex lift not necessary for delta winged planforms?

As I understand it, vortex lift is very helpful for high AoA to allow reasonable landing speeds, but drones may not care as much about that (the controlling computer has much faster reflexes than a human, hence ability to fly a statically unstable airframe).

UAVs also don't have anyone on board to complain about rougher landings or hard breaking that may come from higher landing speeds.

It may be that this particular drone has been optimised for "time on station", requiring it to operate at the lowest possible airspeed which would involve a high angle of attack, or even a STOL capability. This design decision may also be prompted by increased internal volume for fuel, sensors, novel antenna shapes or weapons. Everything about aircraft design is a compromise between competing criteria.


text:
Why is “L Fuel Low” illuminating on my Cessna that is full of gas?
I fly a fuel injected Cessna 172SP 200 hp. Tanks were just about full today and every 5 seconds or so in flight, I would get “L fuel low” and the left fuel indicator would drop to 0.
I’ve consulted with a few people and get differing answers. Some say it’s due to the blue dye in the tank? Some say bad sensor? Any thoughts?

Ask the people that say blue dye in the tank to explain themselves.  It’s probably a faulty sensor, but you should find a local A&P you can trust and avoid troubleshooting via random internet strangers.

Airplane fuel level sensors are notoriously bad, half the airplanes I fly have them marked as INOP, instead we dip the tanks before flight and keep track of the burn rate.

Fuel dye has nothing to do with it.
The fuel sender in light airplanes is usually an automotive-style float operated potentiometer, a rotary variable resistor, like a volume knob on a radio.  Inside is an arm, or wiper, that sweeps around a resistive conductor in an arc, providing a variable output voltage for the fuel gauge.  There will be a voltage range for the potentiometer output to the gauge with 28 vdc going in, say, 22 vdc for full, and 4 vdc or less for empty. The low fuel light will be set to come on when the voltage is some margin above that, say, 8 vdc or less or something like that.
Sometimes the resistance element inside the pot is a kind of circular wire coil on a card, sometimes it's a resistive coating on a flat surface.  If part of the resistor is worn to the point where there is an open circuit when the wiper passes the worn spot, the voltage going to the fuel gauge drops to zero, until the arm moves off the worn spot and the voltage is picked up again.
The low fuel light coming on plus the gauge dropping to zero suggests that the voltage signal is dropping out upstream of the fuel gauge and sensing circuit for the low fuel light.  Could be due to a worn spot in the pot as the float moves when fuel sloshes around in the tank, or it could be an intermittent open in the wiring.  If the problem goes away when the fuel level drops to some lower amount where the potentiometer no longer contacts the worn spot, that would tend to confirm that possibility.
If I was the mechanic I'd try to duplicate the fault by sticking something in the tank that can move the float up and down and observe the result.  If everything works normally, it's more likely to be a loose connection somewhere giving an intermittent open circuit. Check wiring, and if ok swap out the level sender and see if the problem goes away.

A great answer! Some older automotive airflow sensors work on the same principle, and they can develop the same issues, causing rough running at certain throttle levels. There are ways to resurface these plates (a pencil will work in some cases) or work around them by extending the sensor arm contact point to an unworn spot, but that's only really worth it when the part is expensive or hard to find.

Thanks. It tends to affect rental airplanes more because they spend most of their time between 3/4 and full tanks as each renter "fills 'er up" just to be safe and flies around for an hour burning off the top quarter until the next renter fills it again.  In the commercial world, where fuel in excess of minimum safety requirements is effectively ballast that replaces revenue, there is way more variability in fuel quantity over time.


text:
What does this wind symbol mean?
At https://www.aviationweather.gov on the Forecasts puldown, then the Aviation Forecasts selection, the chart opens and often shows what looks like a letter R with a tail in the lower right, in different colors.  The legend does show the symbol. What does it indicate?

related, with possibly the answer: https://aviation.stackexchange.com/q/3037/1467

The Info button (top right) takes you to an explanation site, which in turn links you to
https://www.AviationWeather.gov/metar/symbol
Which explains the Symbol (under the section Fog or special weather) as Thunderstorm

Excellent!  I guess I need to poke around there more, I hadn't even tried the Info pages at all. Thanks.